This invention relates to a laminate for a safety glazing and more particularly to reducing optical defects in such a glazing containing such a laminate.
Angstrom-thick (or thicker) layers of metals, metal compounds and the like in windows to reflect heat-producing infra-red solar radiation while transmitting significant cooler visible light are well known. Temperature increase is reduced within an area delimited by one or more of such windows. These layers usually arranged in sequence as stacks are carried by an appropriate transparent planar substrate such as biaxially stretched, thermoplastic polyethylene terephthalate film (PET) or equivalent material (hereinafter occasionally collectively called "metallized film"). One form known as an interference filter comprises at least one layer of reflective metal sandwiched between reflection-suppressing or anti-reflective dielectric layers. Likewise known is heating the metal layer by electrical conductance to provide defrost or deice and/or defog capability.
When a metallized film such as an interference filter is combined with glass in a laminated safety glazing, for example in a vehicle windshield, a shock-dissipating interlayer of plasticized polyvinyl butyral (PVB) is usually included to absorb a blow from the head of an occupant within the vehicle or from a foreign object from outside the vehicle without penetrating the windshield. As disclosed in U.S. Pat. No. 4,782,216, col. 3, lines 44-56, in a preferred structure a metal/dielectric stack on a flexible, transparent film substrate such as PET is encapsulated within two layers of plasticized PVB, one layer against the substrate and the other against the interference filter. This multi-layered laminate with PVB as the outer layers is then laminated between two rigid transparent members such as glass to form the safety glazing which, by virtue of the presence of the interference filter, controls solar radiation transmission.
A persistent optical quality defect particularly noticeable at oblique viewing angles can occur in such safety glazings as a visually apparent, isotropic, wave-like reflected image estimated to have an amplitude of about 0.1 to 0.5 mils and a wavelength of 100-300 mils. When pronounced, this defect renders the glazing commercially unacceptable. It is referred to hereinafter by the term "applesauce".
As disclosed in commonly owned, copending U.S. application Ser. No. 07/278,576, filed Dec. 1, 1988, by carefully designing the interference filter to minimize visible reflectivity at the PVB/metallized film interface and providing a neutral or green (vs red) color, the severity of this applesauce defect is reduced but not eliminated and frequently not reliably reduced to an acceptable level. While moderately successful, these approaches are cosmetic in reducing the visibility of but not dealing with or eliminating the applesauce optical defect per se.
Another attempt to deal with this optical defect is described in U.S. Pat. No. 4,465,736 where a solar reflecting metal/dielectric stack, therein called a coating, is on a special substrate designed to be heat shrinkable within certain carefully prechosen limits. The problem with this is the potential for unequal thermal shrinkage of the substrate and coating thereon, which can cause the coating to become discontinuous. Discontinuities in the coating can result in degradation in both electrical (i.e. higher resistance) and optical properties (i.e. an increase in the scattering of visible light). Moreover, according to this '736 patent, commercially available PVB sheet seemingly must first be specially heat treated before use with the metallized film.
Rather than mask the applesauce defect in a safety glazing, it would be desirable to eliminate it altogether or at least significantly reduce its severity.